In recent years, fuel cells that have excellent generation efficiency and emit no carbon dioxide are being developed for global environment protection. Such a fuel cell generates electricity from hydrogen and oxygen through an electrochemical reaction. The fuel cell has a sandwich-like basic structure, and includes an electrolyte membrane (ion-exchange membrane), two electrodes (fuel electrode and air electrode), gas diffusion layers of oxygen (air) and hydrogen, and two separators.
Fuel cells are classified as phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, alkaline fuel cells, and polymer electrolyte fuel cells (PEFC: proton-exchange membrane fuel cells or polymer electrolyte fuel cells) according to the type of electrolyte membrane used, which are each being developed.
Of these fuel cells, polymer electrolyte fuel cells have, for example, the following advantages over other fuel cells.
(a) The fuel cell operating temperature is about 80° C., so that electricity can be generated at significantly low temperature.
(b) The fuel cell body can be reduced in weight and size.
(c) The fuel cell can be started promptly, and has high fuel efficiency and power density.
Polymer electrolyte fuel cells are therefore expected to be used as power sources in electric vehicles, home or industrial stationary generators, and portable small generators.
A polymer electrolyte fuel cell extracts electricity from hydrogen and oxygen via a polymer membrane. As illustrated in FIG. 1, a membrane-electrode joined body 1 is sandwiched between gas diffusion layers 2 and 3 (for example, carbon paper) and separators (bipolar plates) 4 and 5, forming a single component (a single cell). An electromotive force is generated between the separators 4 and 5.
The membrane-electrode joined body 1 is called a membrane-electrode assembly (MEA). The membrane-electrode joined body 1 is an assembly of a polymer membrane and an electrode material such as carbon black carrying a platinum catalyst on the front and back surfaces of the membrane, and has a thickness of several 10 μm to several 100 μm. The gas diffusion layers 2 and 3 are often integrated with the membrane-electrode joined body 1.
In the case of actually using polymer electrolyte fuel cells, several tens to hundreds of single cells such as the above are typically connected in series to form a fuel cell stack and put to use.
The separators 4 and 5 are required to function not only as
(a) partition walls separating single cells, but also as
(b) conductors carrying generated electrons,
(c) air passages 6 through which oxygen (air) flows and hydrogen passages 7 through which hydrogen flows, and
(d) exhaust passages through which generated water or gas is exhausted (the air passages 6 or the hydrogen passages 7 also serve as the exhaust passages).
The separators therefore need to have excellent durability and electric conductivity.
Regarding durability, about 5000 hours are expected in the case of using the polymer electrolyte fuel cell as a power source in an electric vehicle, and about 40000 hours are expected in the case of using the polymer electrolyte fuel cell as a home stationary generator or the like. Since the proton conductivity of the polymer membrane (electrolyte membrane) decreases if metal ions are eluted due to corrosion, the separators need to be durable for long-term generation.
Regarding electric conductivity, the contact resistance between the separator and the gas diffusion layer is desirably as low as possible, because an increase in contact resistance between the separator and the gas diffusion layer causes lower generation efficiency of the polymer electrolyte fuel cell. A lower contact resistance between the separator and the gas diffusion layer contributes to better power generation property.
Polymer electrolyte fuel cells using graphite as separators have already been in practical use. The separators made of graphite are advantageous in that the contact resistance is relatively low and also corrosion does not occur. The separators made of graphite, however, easily break on impact, and so are disadvantageous in that the size reduction is difficult and the processing cost for forming gas flow passages is high. These drawbacks of the separators made of graphite hinder the widespread use of polymer electrolyte fuel cells.
Attempts have been made to use a metal material as the separator material instead of graphite. In particular, various studies have been conducted to commercialize separators made of stainless steel, titanium, a titanium alloy, or the like for enhanced durability and lower contact resistance.
For example, JP H8-180883 A (PTL 1) discloses a technique of using, as separators, a metal such as stainless steel or a titanium alloy that easily forms a passive film. With the technique disclosed in PTL 1, however, the formation of the passive film causes an increase in contact resistance, and leads to lower generation efficiency. The metal material disclosed in PTL 1 thus has problems such as high contact resistance as compared with the graphite material.
JP H10-228914 A (PTL 2) discloses a technique of plating the surface of a metal separator such as an austenitic stainless steel sheet (SUS304) with gold to reduce the contact resistance and ensure high output. However, gold plating incurs higher cost.
JP 2000-328200 A (PTL 3) and JP 2007-12634 A (PTL 4) disclose techniques of exposing a metal boride at the surface of stainless steel to reduce the contact resistance. These techniques, however, require the addition of a large amount of B, C, and the like as a steel component, so that workability decreases. Besides, since a large precipitate is exposed at the steel surface, cracking, rough surface, and the like tend to originate from the coarse precipitate when working the steel into a separator shape. Furthermore, the reduction in contact resistance is insufficient.